An emergency descent made by an Air China passenger plane after the cabin lost pressure on July 10, 2018, had experts scratching their heads. Now, a preliminary investigation by China’s aviation regulator, amid intense Chinese news media reporting on the incident, has attributed the event to the co-pilot smoking an e-cigarette in the cockpit during the flight.

What happened: Air China Flight CA106 was on route from Hong Kong to Dalian in China, when the Boeing 737, carrying 153 passengers and nine crew members onboard, suddenly dropped, prompting the cabin's oxygen masks to be deployed. The jetliner descended from 35,000 feet to 10,000 feet in just 10 minutes. It then ascended and continued to its destination at a peak altitude of 26,600 feet.

What came after: according to Reuters, the decision to climb again and continue the flight, rather than head to a nearby airport, was described as unusual by industry experts. The oxygen masks had already been deployed and there was a risk of another decompression event after the one-time supply of about 12-20 minutes from the oxygen masks was used up.

Although the plane landed safely and there were no injuries as a result of the incident, the northeast bureau of the Civil Aviation Administration of China (CAAC) launched an investigation. Its initial results found that the co-pilot was smoking an e-cigarette during the flight.

Investigators said that the co-pilot tried to hide the fact that he was smoking but accidentally shut off the air-conditioning, causing oxygen levels to fall, BBC News reports. “Smoke diffused into the passenger cabin and relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen,” Qiao Yibin, an official of the CAAC’s aviation safety office, was quoted as saying at CAAC’s news conference by state-owned China News.

Shutting off the air conditioning units triggered an alarm and prompted the crew to perform an emergency pressure relief procedure, which then released the cabin’s oxygen masks. After the rapid descent, the crew realized the problem and reactivated the air conditioning, allowing cabin pressure to return to normal, and the plane to climb and continue on its path.

What happens now: Qiao promised to hand down "severe punishment in accordance with laws and regulations," if the regulator's final conclusion on the incident matches its initial finding, CNN News reports. Meanwhile, the state-backed Air China, on its part, said it would terminate the contracts of the flight crew involved in the incident, and suggested the CAAC to cancel their licenses.

According to Reuters, in 2006, China’s aviation regulations, which prohibit flight crew from “smoking on all phases of operation”, also banned passengers from smoking e-cigarettes on flights.